1. Field of the Invention
The invention relates to submersible electric centrifugal pumps.
2. Description of the Prior Art
Submersible electric cenrifugal pumps are commonly used downhole in a well, to pump production fluid to the surface. Typically, such pumps operate most reliably and efficiently at flow rates of about 600 barrels per day. Many wells, however, produce only small quantities of oil, on the order of 150 barrels per day, or less.
At less than optimum flow rates, a pump is not only less efficient, but motor cooling is reduced in proportion to reduced fluid flow. The danger of motor failure through overheating becomes significant.
Recirculation of pump output has been used to achieve the necessary flow rate through the pump. In one system, described in U.S. Patent Application Ser. No. 039,775, filed May 17, 1979, now U.S. Pat. No. 4,294,573 assigned to the assignee of this Application, a portion of the output of one or more electrical centrifugal pumps is recirculated as power fluid through a jet pump. The jet pump aspirates production fluid, and its entire combined output flows to the centrifugal pump inlet. It may be preferable that the inlet of the jet pump be fed by an auxiliary centrifugal pump, to prevent cavitation. The system adapts pumps having relatively high flow requirements to wells producing at a relatively low rate. Thus the flow rate through the centrifugal pump, including recirculated fluid, is consistent with optimal operation of the pump, while the small portion of the pump output diverted to the surface may be equal to the ideal well production rate.
Such an assembly of a jet pump and electrically powered centrifugal pumps comprises pump components chosen for the expected well conditions. When it is desired to change the pumping characteristics of the pump assembly, as the well is pumped off, for example, it has been necessary to substitute another jet pump in the downhole assembly. To facilitate the substitution of a different size jet pump to suit the changed well requirements, the jet pump is disposed uppermost in the assembly, even though it is intermediate in the fluid flow path. Accordingly, while the characteristics of the pump assembly could be tailored to well requirements without removal of the complete assembly, a time consuming retrieval and substitution operation for the jet pump component was necessary.